Elbow Sealing
Probably because they've used RTV before and left a lip, leaving an uneven seating surface for the o-ring that eventually leaks, or failed to 'Clean Out' the holes properly, thereby achieving the same result. Lolz...

Anyways, I agree, no extra sealing required, they work just fine as they're made, if plastic is used, then they should be changed out on regular maintenance interval, especially for the few $ they cost...
Pre-lubing the rings with a little silicone or likewise will aid seating and keep rings in shape longer.